Human cytomegalovirus (HCMV) is one of the most common opportunistic infectious agents in AIDS patients. Viral infections in the spleen are believed to contribute to pathogenesis and virulence of systemic infections since highly active viral replication in the spleen, common in immunocompromised hosts, usually leads to lesions in the organ and impairs its function for control of viral infection. During primary infection, the viruses produced in the spleen disseminate to other organs and represent one of the major sources for subsequent infection of many organs. Moreover, the spleen is also a site for viral persistent and latent infections. Thus, CMV replication in the spleen is a major determinant of viral virulence and pathogenesis. However, little is currently known about the mechanism of how CMV replicates in the spleen. Equally elusive is the nature of viral determinants required for CMV infection in the organ. Using murine cytomegalovirus (MCMV) infection in immunodeficient animals as a model system, the proposed research is to study viral genes required for CMV replication in spleen and to investigate the roles of these viral determinants in supporting CMV infections in splenic tissues and cells. We have recently developed a novel approach to construct MCMV mutants and have isolated two novel mutants that are defective in growth in spleens and exhibit little virulence in killing immunodeficient animals. One mutant was attenuated to grow in splenic cells while the other failed to modulate NK cell-mediated response in spleens. In the proposed research, we will first study how these two mutants are defective in infecting spleen tissues and cells. We will then characterize the mutated ORFs, which encode MCMV determinants for viral infection in spleen. Furthermore, host factors that potentially interact with these viral ORFs will be identified and their interactions with viral proteins will be examined. Finally, we will investigate the mechanism of how the identified viral determinants function in supporting MCMV infections in spleens, including in modulating dendritic cells and in blocking NKG2D-ligand interactions. These studies will characterize viral determinants for infection in spleens and elucidate the role of these genes in enhancing viral virulence and causing CMV systemic infection. Studying viral virulence factors and understanding their roles in CMV pathogenesis will provide insight into developing novel approaches for the treatment of CMV-associated complications, including those associated with AIDS patients. PUBLIC HEALTH RELEVANCE: Human cytomegalovirus (CMV) causes one of the most common opportunistic infections encountered in AIDS patients, and viral infection in spleen is primarily responsible for CMV virulence in immunodeficient hosts. The objective of the proposed studies is to identify the cytomegalovirus (CMV) genes that are important for viral infection in the spleen, and to study the mechanism of viral replication in the spleen tissue. Our study will facilitate the development of new drugs and novel approaches for treating CMV infection by blocking the expression and function of these identified genes.